Electrophoretic medium

ABSTRACT

An electrophoretic medium comprises a plurality of charged particles disposed in a fluid. The fluid comprises at least about 75, and preferably at least about 95, percent by weight of a hydrocarbon selected from monounsaturated nonenes, nonane and methyloctane. The electrophoretic medium is especially useful in microcell electrophoretic media comprising a substrate having a plurality of cavities, and a sealing layer closing the open ends of the cavities, the cavities being filled with the electrophoretic medium.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/115,997, filed on Aug. 29, 2018, and published on Feb. 28, 2019 asU.S. Patent Publication No 2019/0064625, which claims priority to U.S.Provisional Application Ser. No. 62/551,959, filed on Aug. 30, 2017, thecontents of which are incorporated by reference herein in theirentireties.

BACKGROUND OF INVENTION

This invention relates to an electrophoretic medium. More specifically,this invention relates to an electrophoretic medium comprising aspecific type of fluid, and to microcell and other electrophoreticdisplays comprising such an electrophoretic medium.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

One type of display, which has been the subject of intense research anddevelopment for a number of years, is the particle-based electrophoreticdisplay, in which a plurality of charged particles move through a fluidunder the influence of an electric field. Electrophoretic displays canhave attributes of good brightness and contrast, wide viewing angles,state bistability, and low power consumption when compared with liquidcrystal displays. Nevertheless, problems with the long-term imagequality of these displays have prevented their widespread usage. Forexample, particles that make up electrophoretic displays tend to settle,resulting in inadequate service-life for these displays.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT), E Ink Corporation, E InkCalifornia, LLC. and related companies describe various technologiesused in encapsulated and microcell electrophoretic and otherelectro-optic media. Encapsulated electrophoretic media comprisenumerous small capsules, each of which itself comprises an internalphase containing electrophoretically-mobile particles in a fluid medium,and a capsule wall surrounding the internal phase. Typically, thecapsules are themselves held within a polymeric binder to form acoherent layer positioned between two electrodes. In a microcellelectrophoretic display, the charged particles and the fluid are notencapsulated within microcapsules but instead are retained within aplurality of cavities formed within a carrier medium, typically apolymeric film. The technologies described in these patents andapplications include:

-   -   (a) Electrophoretic particles, fluids and fluid additives; see        for example U.S. Pat. Nos. 5,961,804; 6,017,584; 6,120,588;        6,120,839; 6,262,706; 6,262,833; 6,300,932; 6,323,989;        6,377,387; 6,515,649; 6,538,801; 6,580,545; 6,652,075;        6,693,620; 6,721,083; 6,727,881; 6,822,782; 6,831,771;        6,870,661; 6,927,892; 6,956,690; 6,958,849; 7,002,728;        7,038,655; 7,052,766; 7,110,162; 7,113,323; 7,141,688;        7,142,351; 7,170,670; 7,180,649; 7,226,550; 7,230,750;        7,230,751; 7,236,290; 7,247,379; 7,277,218; 7,286,279;        7,312,916; 7,375,875; 7,382,514; 7,390,901; 7,411,720;        7,473,782; 7,532,388; 7,532,389; 7,572,394; 7,576,904;        7,580,180; 7,679,814; 7,746,544; 7,767,112; 7,848,006;        7,903,319; 7,951,938; 8,018,640; 8,115,729; 8,199,395;        8,257,614; 8,270,064; 8,305,341; 8,361,620; 8,363,306;        8,390,918; 8,582,196; 8,593,718; 8,654,436; 8,902,491;        8,961,831; 9,052,564; 9,114,663; 9,158,174; 9,341,915;        9,348,193; 9,361,836; 9,366,935; 9,372,380; 9,382,427; and        9,423,666; and U.S. Patent Applications Publication Nos.        2003/0048522; 2003/0151029; 2003/0164480; 2003/0169227;        2003/0197916; 2004/0030125; 2005/0012980; 2005/0136347;        2006/0132896; 2006/0281924; 2007/0268567; 2009/0009852;        2009/0206499; 2009/0225398; 2010/0148385; 2011/0217639;        2012/0049125; 2012/0112131; 2013/0161565; 2013/0193385;        2013/0244149; 2014/0011913; 2014/0078024; 2014/0078573;        2014/0078576; 2014/0078857; 2014/0104674; 2014/0231728;        2014/0339481; 2014/0347718; 2015/0015932; 2015/0177589;        2015/0177590; 2015/0185509; 2015/0218384; 2015/0241754;        2015/0248045; 2015/0301425; 2015/0378236; 2016/0139483; and        2016/0170106;    -   (b) Capsules, binders and encapsulation processes; see for        example U.S. Pat. Nos. 6,922,276 and 7,411,719;    -   (c) Microcell structures, wall materials, and methods of forming        microcells; see for example U.S. Pat. Nos. 6,672,921; 6,751,007;        6,753,067; 6,781,745; 6,788,452; 6,795,229; 6,806,995;        6,829,078; 6,833,177; 6,850,355; 6,865,012; 6,870,662;        6,885,495; 6,906,779; 6,930,818; 6,933,098; 6,947,202;        6,987,605; 7,046,228; 7,072,095; 7,079,303; 7,141,279;        7,156,945; 7,205,355; 7,233,429; 7,261,920; 7,271,947;        7,304,780; 7,307,778; 7,327,346; 7,347,957; 7,470,386;        7,504,050; 7,580,180; 7,715,087; 7,767,126; 7,880,958;        8,002,948; 8,154,790; 8,169,690; 8,441,432; 8,582,197;        8,891,156; 9,279,906; 9,291,872; and 9,388,307; and U.S. Patent        Applications Publication Nos. 2003/0175480; 2003/0175481;        2003/0179437; 2003/0203101; 2013/0321744; 2014/0050814;        2015/0085345; 2016/0059442; 2016/0004136; and 2016/0059617;    -   (d) Methods for filling and sealing microcells; see for example        U.S. Pat. Nos. 7,144,942 and 7,715,088;    -   (e) Films and sub-assemblies containing electro-optic materials;        see for example U.S. Pat. Nos. 6,982,178 and 7,839,564;    -   (f) Backplanes, adhesive layers and other auxiliary layers and        methods used in displays; see for example U.S. Pat. Nos.        7,116,318 and 7,535,624;    -   (g) Color formation and color adjustment; see for example U.S.        Pat. Nos. 7,075,502 and 7,839,564;    -   (h) Methods for driving displays; see for example U.S. Pat. Nos.        7,012,600 and 7,453,445; and    -   (i) Applications of displays; see for example U.S. Pat. Nos.        7,312,784 and 8,009,348.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, U.S. Pat. Nos.5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and6,184,856. Dielectrophoretic displays, which are similar toelectrophoretic displays but rely upon variations in electric fieldstrength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.Other types of electro-optic displays may also be capable of operatingin shutter mode. Electro-optic media operating in shutter mode may beuseful in multi-layer structures for full color displays; in suchstructures, at least one layer adjacent the viewing surface of thedisplay operates in shutter mode to expose or conceal a second layermore distant from the viewing surface.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

One critical factor in the performance of an electrophoretic medium isthe choice of the fluid (sometimes referred to in the literature as the“suspending fluid”, dispersing fluid”, “dielectric fluid” or sometimes,rather illogically, “solvent” or “solvent mixture”). The chiefdifficulty lies in the number of desirable criteria for such a fluid andthe difficulty of satisfying all of the criteria at the same time. Amongsuch criteria are:

-   -   a) low viscosity, to permit rapid movement of the charged        particles in the applied electric field;    -   b) low dielectric constant to allow the charged particle to        experience a high electric field and thus move as rapidly as        possible;    -   c) chemical stability against, for example, radiation to which        the medium may be exposed, particularly if use outdoors, and        gases, especially oxygen, which may dissolve in the fluid;    -   d) chemical compatibility with a wide variety of electrophoretic        particles and other components present in the fluid, for example        charge control agents;    -   e) chemical compatibility with wall materials used to        encapsulate the electrophoretic medium and/or continuous phase        materials surrounding the electrophoretic medium;    -   f) low water absorption, since the behavior of many        electrophoretic particles is affected by even minor amounts of        water present in the fluid;    -   g) high density, to reduce the tendency for electrophoretic        particles, which are often relatively dense metal oxide        pigments, to settle out of the fluid; and    -   h) high electrical resistivity to reduce current flow through        the medium and hence reduce power consumption.

Additional criteria apply to electrophoretic media intended for use inmicrocells. Microcell cell displays (see the patents and applicationslisted in sub-Paragraph 5(c) above) are typically produced by aso-called “fill and seal” process, which comprises forming a series ofrecesses in a substrate, filling the electrophoretic medium into thesecells and forming a sealing layer over the filled cells. Since it isessential that the cells be completely filled with the medium (to avoidair bubbles within the cells) but not overfilled (which would causedifficulties in securing the sealing layer to the cell walls), the fluidshould have a low vapor pressure to reduce evaporation during thefilling of the cells. (It should be noted that this problem with fluidevaporation is of much less concern in other applications of similarcompositions; for example, in xerographic toners, where thenon-encapsulated, single use toner composition can tolerate significantfluid evaporation during use.) It is also desirable that the propertiesof the fluid be chosen to minimize a problem known as “sag in”, which isillustrated in FIG. 1 of the accompanying drawings. FIG. 1A is aschematic cross-section through a filled microcell medium (generallydesignated 100) comprising a substrate 102 on which are formed a seriesof microcells having bases 104 and cell walls 106, the cells beingfilled with an electrophoretic medium 108. The cells are sealed by asealing layer 110. As illustrated in FIG. 1A, the width of the cells issubstantially greater than the height of the cell walls 106, so thatthere is a substantial unsupported “run” of the sealing layer 110between adjacent cell walls 106, and, depending upon the mechanicalproperties of the electrophoretic medium 108 and the sealing layer 110,there is a tendency for the sealing layer 110 to “sag in” in the centralpart of each cell, so that the depth of the electrophoretic medium inthe central part of each cell in less than the height of the cell walls106. Excessive sag in can lead to broken or cracked sealing layer, theconsequent loss of fluid from the affected cells and hindered ornon-existent switching. Moderate sag in can lead to surface roughness ofthe sealing layer, with associated optical problems due to lightscattering, and may also cause problems with void creation duringlamination processes, such as those typically used to attach the sealinglayer to a backplane or other electrode structure. An electrophoreticdevice according to the present invention is illustrated in FIG. 1B. Theelectrophoretic device 150 of FIG. 1B comprises a layer ofelectrophoretic medium 108 and at least one electrode 120 disposedadjacent the layer of electrophoretic medium. The electrophoretic mediumcomprises two types of charged particles that are oppositely charged(114) in a fluid (156). The aforementioned problems regarding choice offluids for use in electrophoretic media have frequently beenacknowledged in the literature but are far from solved. In many cases,the prior art recites lengthy lists of possible fluids with no guidanceas to the optimum fluid and without provided any exemplification of theproperties of the fluids, apparently leaving the reader to optimize thefluid for each new electrophoretic medium.

The aforementioned problems regarding choice of fluids for use inelectrophoretic media have frequently been acknowledged in theliterature but are far from solved. In many cases, the prior art reciteslengthy lists of possible fluids with no guidance as to the optimumfluid and without provided any exemplification of the properties of thefluids, apparently leaving the reader to optimize the fluid for each newelectrophoretic medium.

For example, U.S. Pat. No. 5,453,121 describes an “ink jet inkcomposition wherein the liquid is selected from the group of aliphatichydrocarbons, aromatic hydrocarbons, chlorinated solvents, polysiloxanesor mixtures thereof or is a vegetable oil selected from olive oil,safflower oil, sunflower oil, soya oil and linseed oil or mixturesthereof.” These solvents are chosen to allow droplet formation with verylittle solvent, though the only relevant physical property of theliquids discussed is their electrical resistance.

U.S. Pat. No. 5,457,002 describes carrier fluids for toners to be usedin electrophotography and reports that low volatility and low viscositycan be obtained using trimers of C₉ to C₁₁ α olefins. These materialswould not be appropriate for use in electrophoretic displays because oftheir relatively high viscosity (˜20 centistokes at 40° C.).

U.S. Pat. No. 5,411,656 describes an electrophoretic medium containingthe sterically strained alkene 5-ethylidene-2-norbornene (CAS No.16219-75-3). However, this compound is present in only a minorproportion in the fluid, being intended as an additive for chlorine gasabsorption rather than as the main fluid.

U.S. Pat. No. 7,079,305 and others of E Ink Corporation teach thatsuitable solvents for encapsulated electrophoretic media should have lowdielectric constant, high volume resistivity, low viscosity (less than 5centistokes), low toxicity, low water solubility, high specific gravity,high boiling point and low refractive index, as well as density matchingand chemical compatibility with the electrophoretic particles, stating.“Organic solvents, such as halogenated organic solvents, saturatedlinear or branched hydrocarbons, silicone oils, and low molecular weighthalogen-containing polymers are some useful suspending fluids.” Column15, line 65 to column 16, line 27 of U.S. Pat. No. 7,079,305 contains along list of possible fluids, including aliphatic and aromatichydrocarbons, halocarbons, silicones and halogenated oligomers andpolymers.

U.S. Pat. No. 7,545,557 describes an electrophoretic medium comprising afluid that has chemical inertness, density matching to the particles,chemical compatibility with the particles, low dielectric constant, andlow viscosity (for example 0.5 to 5 centistokes). Solvents thatpresumably meet these criteria, as described in the specification,include saturated linear or branched hydrocarbons, silicone oils, andlow MW halogenated polymers. No guidance is provided on specificpreferred fluids.

U.S. Pat. No. 7,679,814 describes the use of mixtures of partiallyhydrogenated aromatic hydrocarbons and terpenes for reduced haze invariable transmission electrophoretic media.

U.S. Pat. No. 8,786,935 teaches that fluids for electrophoretic displayspreferably have low viscosity and low dielectric constant, and listshydrocarbons (e. g., Isopar, decalin, 5-ethylidene-2-norbornene,paraffin oil), silicone oils, aromatic hydrocarbons (including toluene,alkylnaphthalene), and halogenated solvents (e.g. Halocarbon Oils fromHalocarbon Product Corp or FC-43 from 3M Company). The patent gives noguidance as to how to select an optimal solvent from this rather lengthylist.

U.S. Pat. No. 8,670,174 describes non-polar solvents for use inhighlighted or multicolor electrophoretic displays and, using languageessentially identical to U.S. Pat. Nos. 5,582,700 and 7,940,450, statesthat suitable solvents may include “C₁₋₃₀ alkanes, C₂₋₃₀ alkenes, C₃₋₃₀alkynes, C₃₋₃₀ aldehydes, C₃₋₃₀ ketones, C₂₋₃₀ ethers, C₂₋₃₀ esters,C₃₋₃₀ thioesters, terpenes, C₂₋₃₀ organosilanes, C₂₋₃₀ organosiloxanesand the like. Such non-polar solvents may be used alone or incombination.” Again, no guidance is provided on how to select an optimalfluid from this long list.

U.S. Pat. Nos. 7,390,901 and 8,361,620 discuss the specific benefits ofhalogenated fluids, including high specific gravity, inertness,insensitivity to humidity, low dielectric constant, low viscosity, andlow vapor pressure. These fluids are useful for particles withhalogenated protective polymers or with fluorinated dyes. However, useof halogenated fluids tends to require use of halogenated auxiliarymaterials, such as charge control agents, and restricts the types ofelectrophoretic particles which can be used, which is a particularproblem in full color displays.

U.S. Pat. No. 9,341,915 describes an electrophoretic fluid comprisingcharged pigment particles dispersed in a mixture of isoparaffins,wherein the mixture comprises isoparaffins having 8, 9 and 10 carbonatoms, and the total percentage of isoparaffins having less than 8carbon atoms and isoparaffins having more than 10 carbon atoms isgreater than 0% and less than 20% of the mixture.

Empirically, it has been found that, although Isopar E gives goodresults in encapsulated electrophoretic displays, it gives poor resultsin microcell displays due to high fluid loss and sag in. Isopar G, whichhas a lower vapor pressure than Isopar E, results in less fluid loss andlower sag in, but the higher viscosity of Isopar G results in slowerswitching speeds.

It has now been found that a limited class of C₉ fluids give excellentresults in microcell electrophoretic displays with low fluid loss andlow sag in. These fluids may also be useful in other types ofelectrophoretic media.

SUMMARY OF INVENTION

Accordingly, this invention provides an electrophoretic mediumcomprising a plurality of charged particles disposed in a fluid, whereinthe fluid comprises at least about 75 percent by weight of a hydrocarbonselected from the group comprising monounsaturated nonenes, nonane andmethyloctane.

The fluid present in the electrophoretic medium of the present inventionmay comprise at least about 90, and preferably at least about 95,percent by weight of the specified hydrocarbons. In a preferredembodiment, the fluid may consist essentially of the specifiedhydrocarbons. The fluid may comprise, for example, any one or more of“tripropylene” (a commercial material comprising a mixture of variousisomers of nonenes), methyloctene, dimethylheptene, non-2-ene, andnonane (i.e., n-nonane and methyloctane).

As already indicated, the electrophoretic medium of the presentinvention is especially intended for use in microcell displays. Suchmicrocell displays comprise a plurality of cavities formed in asubstrate, and a sealing layer closing the open ends of the cavities,the cavities being filled with the electrophoretic medium of theinvention. However, the electrophoretic medium of the present inventionmay be used in other types of both encapsulated and non-encapsulatedmedia. Thus, the present invention extends to an electrophoretic displaycomprising a layer of an electrophoretic medium of the invention and atleast one electrode disposed adjacent the layer of electrophoreticmedium and arranged to apply an electric field thereto. Typically, suchan electrophoretic display will have at least two electrodes disposed onopposed sides of the layer of electrophoretic material. The layer ofelectrophoretic medium may be unencapsulated or encapsulated. Aspreviously described, in addition to microcell media, encapsulated mediainclude media comprising a plurality of capsules, each of which itselfcomprises an internal phase containing the charged particles in thefluid, and a capsule wall surrounding the internal phase. Typically, thecapsules are themselves held within a polymeric binder to form acoherent layer. The layer of electrophoretic medium may also be ofpolymer-dispersed type described above, with the charged particles andthe fluid being present as a plurality of discrete droplets surroundedby a continuous phase of a polymeric material.

The (electrically) charged particles used in the electrophoretic mediumof the present invention may be of any of the types used in prior artelectrophoretic media, as described for example in the aforementioned EInk and MIT patents and applications. Thus, for example, theelectrophoretic medium may comprise only a single type of chargedparticles. Alternatively, the electrophoretic medium may comprise twotypes of particles bearing charges of opposite polarity. Full colordisplays may contain more than two types of charged particles; see, forexample, U.S. Pat. No. 9,922,603, which describes electrophoretic mediacontaining six different types of charged particles all having differingcolors. Typically, the charged particles will carry polymeric coatingsas described, for example, in U.S. Pat. Nos. 6,822,782 and 9,688,859.

The electrophoretic media of the present invention may also containvarious additives as used in prior art electrophoretic media. Typically,the electrophoretic media will contain a charge control agent whichserves to control the charge on the particles. The medium may alsocontain a polymer in the fluid to increase the bistability of themedium; see U.S. Pat. No. 7,170,670.

BRIEF DESCRIPTION OF DRAWINGS

As already indicated, FIG. 1A of the accompanying drawings is aschematic cross-section through a filled prior art microcell medium.FIG. 1B illustrates a schematic of an electrophoretic device accordingto the present invention.

FIG. 2 is a graph plotting the vapor pressure of various hydrocarbons at87° C. against their viscosity at 20° C.

FIG. 3 is a graph similar to FIG. 3 but plotting the vapor pressure ofvarious commercial solvents at 20° C. against their viscosity at thesame temperature.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, the present invention provides an electrophoreticmedium comprising a plurality of charged particles disposed in a fluid,wherein the fluid comprises at least about 75 percent by weight of ahydrocarbon selected from the group comprising monounsaturated nonenes,nonane and methyloctane. It has been found that this relatively narrowclass of hydrocarbon fluids provide a nearly ideal combination of lowconductivity, low viscosity, and low vapor pressure. These fluids werechosen to give unexpected improvements to both vapor pressure andviscosity over electrophoretic fluids described in the prior art, andare particularly good choices for fluids in microcell media produced bya fill and seal process.

As already indicated, fluids for use in microcell electrophoreticdisplays need to possess low conductivity, low viscosity, and low vaporpressure. Low conductivity can be ensured using a hydrocarbon fluid.However, because vapor pressure and viscosity are inversely correlatedfor most liquids, selection of a suitable fluid always involves sometrade-off between these properties. FIG. 2 of the accompanying drawingsshows the potential trade-offs by plotting calculated values forviscosity at 20° C. against vapor pressure at 87° C. for some commonhydrocarbon fluids. Note that alkenes show a better vaporpressure-viscosity relationship than other classes of hydrocarbons,including linear, branched (di-methyl), and cyclic hydrocarbons.Monomethyl substituted linear alkanes also show some advantage.

FIG. 3 is a plot similar to that of FIG. 2 but comparing the vaporpressure of various commercial hydrocarbon solvents at 20° C. againsttheir viscosity at the same temperature. Commonly used electrophoreticsuspending fluids are Isopar G, Isopar E and Isane IP 140 (a mixture ofbranched C₉ hydrocarbons). Isane IP 140 gives vapor pressure-viscositybehavior intermediate between Isopar E and Isopar G. Tripropylene has aneven better viscosity-vapor pressure relationships than Isane IP 140,and is thus predicted to be a superior solvent for use in fill-and-sealmicrocell media; it has lower vapor pressure and lower viscosity thanIsane IP 140.

From FIGS. 2 and 3, it may be expected that the previously specifiedgroup of nonene solvents, including tripropylene, methyloctene,dimethylheptene, non-1-ene, and the like, to be good choices forelectrophoretic fluids. Two other solvents are included as part of thepresent invention, namely: n-nonane and methyloctane.

From the foregoing, it will be seen that the present invention canprovide electrophoretic media with excellent switching speeds, becauseof the low viscosity fluid, and improved manufacturability, because ofthe low vapor pressure of the fluid. Although this invention hasprimarily been described in its application to microcell media, theadvantageous combination of properties provided by the fluids used inthe present invention are useful in other types of electrophoreticmedia.

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the specific embodiments of theinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beinterpreted in an illustrative and not in a limitative sense.

1. An electrophoretic medium comprising at least two types of chargedparticles disposed in a fluid, wherein the fluid comprises at leastabout 75 percent by weight of a hydrocarbon mixture comprisingmonounsaturated nonenes, n-nonane and methyloctane.
 2. Theelectrophoretic medium of claim 1 wherein the fluid comprises at leastabout 90 percent by weight of the hydrocarbon mixture.
 3. Theelectrophoretic medium of claim 2 wherein the fluid comprises at leastabout 95 percent by weight of the hydrocarbon mixture.
 4. Theelectrophoretic medium of claim 1 wherein the monounsaturated nonenesare selected from the group consisting of methyloctene, dimethylheptene,non-2-ene, and mixtures thereof.
 5. The electrophoretic medium of claim1 comprising two types of charged particles bearing charges of oppositepolarity.
 6. The electrophoretic medium of claim 1 comprising more thantwo types of charged particles.
 7. The electrophoretic medium of claim 6comprising six types of charged particles having different colors.
 8. Amicrocell electrophoretic medium comprising a substrate having aplurality of cavities formed therein, and a sealing layer closing theopen ends of the cavities, the cavities being filled with theelectrophoretic medium of claim
 1. 9. The microcell electrophoreticmedium of claim 8, wherein the width of the plurality of the cavities isgreater than the height of the plurality of the cavities.
 10. Anelectrophoretic display comprising a layer of an electrophoretic mediumand at least one light-transmissive electrode disposed adjacent thelayer of electrophoretic medium and arranged to apply an electric fieldthereto, wherein the electrophoretic medium is the electrophoreticmedium of claim
 1. 11. The electrophoretic display of claim 10, whereinthe electrophoretic display comprises two light-transmissive electrodes,and wherein the electrophoretic medium is disposed in between the twolight-transmissive electrodes.
 12. The electrophoretic display of claim10 wherein the electrophoretic medium is confined within a plurality ofcapsules or microcells.
 13. The electrophoretic display of claim 12wherein the capsules are held within a polymeric binder to form acoherent layer.
 14. The electrophoretic display of claim 10 wherein theelectrophoretic medium is present as a plurality of discrete dropletssurrounded by a continuous phase of a polymeric material.